Reciprocating flow ion exchange systems are used in ion exchange processes for the removal and/or recovery of at least part of a component or components from a fluid by contacting the fluid with ion exchange particles. In the context of this patent, ion exchange particles include ion exchange resins (such as a strong acid cation resin), adsorptive particles (such as zeolites), or such other particles as may form the ion exchange bed in an ion exchange vessel.
U.S. Pat. Nos. 3,385,783 and 3,386,914, both issued to Robert F. Hunter, disclose and discuss the prior art process of reciprocating flow ion exchange. In simplified terms, the process involves the passage of the solution to be treated through a bed of ion exchange particles such that at least a part of the component in question is taken up by the particles and removed from solution. The basic cycle consists of two co-current steps and two counter-current steps, with the solution treatment step being the reference for the other steps. The basic steps are (1) solution treatment, (2) counter-current displacement, (3) counter-current regeneration, and (4) co-current displacement. The solution treatment step consists of processing a predetermined amount of solution corresponding to a specific incremental loading of the bed of ion exchange particles. In the counter-current displacement step interstitial solution, remaining in the ion exchange vessel after the solution treatment step, is displaced in a counter-current direction by introducing regenerant in a direction opposite to the flow of the solution being treated. The regeneration (or elution) step entails continuing the flow of regenerant to regenerate the bed of ion exchange particles by removing at least a part of the loaded component from the ion exchange particles. In the co-current displacement step, interstitial regenerant/product is removed from the ion exchange vessel in a co-current direction by introducing solution that is to be treated into the vessel. Once the interstitial regenerant/product has been removed from the vessel, the flow of solution to be treated continues and the cycle starts again. Depending on the application, rinse steps may also be included, usually before and/or after the regeneration step.
Reciprocating flow ion exchange systems can be differentiated from other counter-current ion exchange systems by their characteristic short, fixed bed of ion exchange particles that are tightly packed at all times so that the ion exchange particles cannot be fluidized, the use of fine mesh ion exchange particles, the low incremental loading of the ion exchange particles, relatively fast flow rates, rapid cyclical operation and counter-current regeneration. The reciprocating flow ion exchange process is also characterized by the use of density effects, where possible, to minimize mixing. That is, the more dense fluid is typically fed into the bottom of the vessel, with the less dense fluid entering from the top. The combination of these features act to achieve a highly efficient process that can be undertaken at a relatively low capital and operating cost.
It will be appreciated that in order to maximize the efficiency of the reciprocating flow ion exchange process, it is essential that during the process, (1) fluid being introduced into the vessel is distributed evenly across the end of the ion exchange bed so that the fluid flows to all portions of the ion exchange bed, (2) there is even flow of fluid as it flows through the ion exchange bed (i.e., plug flow) to minimize mixing, and (3) that there is uniform collection of fluid exiting the ion exchange bed to minimize mixing.
Three of the most critical factors that must be addressed in order to ensure even flow distribution across the end of the bed and even flow through the bed include; the need to maintain a substantially constant bed depth throughout the process and to confine and maintain the ion exchange particles forming the bed in a tight, homogeneously packed condition; the need to maintain each end of the ion exchange bed in a substantially flat configuration to allow even flow across the upper and lower ends of the bed; and, the need to evenly distribute the fluid being introduced into the vessel across the relevant end of the bed so that the entire end of the bed is exposed substantially uniformly to the fluid flowing into the vessel resulting in the entire bed being exposed to the fluid at substantially the same time. It will also be appreciated that to ensure uniform collection of fluid as it exits the ion exchange bed each end of the ion exchange bed must be maintained in a substantially flat configuration so that the plug of fluid exits the bed at substantially the same time. There must also be present a means for collecting the fluid as it exits the bed and to direct it out of the vessel.
Others have addressed these concerns through the introduction of various types of apparati that may be used for the purpose of distributing fluid across each end of the bed, collecting the fluid as it exits the bed, holding each end of the bed in a flat configuration, and for confining or holding the ion exchange particles in a fixed position. For example, in U.S. Pat. No. 4,673,507, issued to Craig J. Brown, a particular form of header is described that includes fluid flow passageways that are said to help distribute fluid across the end of the ion exchange bed and to collect the fluid as it exits the end of the bed. In addition, the header is said to maintain the ion exchange bed in a fixed and packed condition.
The prior art devices that have been proposed for use in a reciprocating flow ion exchange system suffer from various inherent problems and disadvantages. For example, the "header" type devices that have been proposed to date can experience problems with distortion and bulging when high pressures are encountered during the process cycle. For example, at an inlet pressure of 70 psi (such as may be encountered in a water deionization process) the header of a prior art vessel holding a 60 inch diameter ion exchange bed will be subjected to approximately 99 tons of pressure (197,920 pounds). Any distortion or bulging of the header presents a significant problem in the process. The ends of the ion exchange bed are no longer maintained in a flat configuration, the ion exchange bed does not maintain a constant depth, fluid cannot be distributed or collected evenly over the entire end of the ion exchange bed and the integrity of gaskets and seals in the vessel may be compromised leading to leakage. In addition, packing uniformity is compromised resulting in possible flow distribution problems.
As a result of such problems it has been shown in practice that the flat header of prior art devices must be significantly reinforced by steel ribs or other reinforcement means where high internal pressures are to be encountered. Such reinforcing adds significantly to both the weight and the cost of the vessel. This condition becomes more prevalent as the diameter of the vessel, and/or the internal pressure, increases.
A second major disadvantage of the prior art concerns the efficiency and cost of fluid distribution devices. That is, in order to achieve full use of the ion exchange bed and maximize process efficiency, elaborate and costly flow distribution plates have been used to help distribute fluid across the upper and lower ends of the ion exchange bed. While such distribution plates assist somewhat in the distribution of the fluid flowing into the vessel by creating passageways through which the fluid is directed, they nevertheless tend to distribute the fluid to the centre of the ion exchange bed before any fluid reaches the outer edges of the bed. This problem also becomes more prevalent as the diameter of the ion exchange bed increases.
A third disadvantage of the prior art devices concerns the packing of the ion exchange bed in order to obtain a relatively tight, homogeneous packing. Others have suggested loading ion exchange particles in the vessel by overfilling the vessel through mounding or heaping the ion exchange particles over the top of the vessel and using a header to compress and hold the ion exchange particles in place. This method is both imprecise and can lead to overly dense areas in the ion exchange bed, resulting in channelling. A channelling bed is a bed containing loose and/or overly dense areas resulting in pathways being created through which a disproportionate amount of fluid passes. It is therefore critical that care be exercised to ensure a uniform packing density when the ion exchange particles are loaded into the ion exchange vessel, thereby forming the ion exchange bed.
The problem becomes even more prevalent where the ion exchange particles are caused to shrink prior to being loaded into the vessel, as suggested by the prior art. The particles may swell considerably during the ion exchange process, resulting in overly dense areas. Furthermore, loading ion exchange particles by heaping them over the top of the vessel and compressing them in place can result in contamination of seals and gaskets which may result in leakage.
A fourth disadvantage of the prior art is the absence of a device or means for readily removing all or a portion of the ion exchange particles from the assembled vessel. Ion exchange particles can become contaminated by fine particulate material or liquids resulting in their becoming fouled and requiring replacement or cleaning. In addition, it may be desirable to change the type or size of the ion exchange particles being used in the process. It will also be appreciated that in any ongoing operation ion exchange particles have a limited useful life and may at some time require replacement. Previously, the ion exchange particles could not be removed from the vessel without first having to physically disconnect the vessel from the ion exchange system and disassemble the top or bottom to gain access to the ion exchange bed. The ion exchange particles were then physically shovelled or scooped out of the vessel. This method of removing the ion exchange particles from the vessel is both extremely messy and time consuming, resulting in significant downtime, lost production and increased costs.